Digital offset printing registration

ABSTRACT

There are disclosed methods and systems for the process of on-press plate imaging, including automatic processes for plate preparation, that compensate for registration and print-length errors (plate loading is performed before imaging). As a result of these processes, the position accuracy is determined by the imaging system. The system of the present invention creates deformed images on the plates during the imaging stage, these deformed images, being such that the separations will be in register (coordinated) after printing.

CROSS REFERENCES TO RELATED APPLICATIONS

This patent application claims priority from and is related to U.S.Provisional Patent Application Ser. No. 60/157,856, entitled DIGITALOFFSET PRINTING REGISTRATION, filed on Oct. 6, 1999, this ProvisionalPatent Application incorporated by reference in its entirety herein.

BACKGROUND OF THE INVENTION

The present invention is directed to lithography, and in particular, tocompensating for deformities in color separations by imagingcorrespondingly deformed images on printing plates, such that theseplates will print the color separations in register.

Conventional lithography processes typically replicate images bytransferring ink from a previously prepared plate onto a substrate,typically paper. In offset printing, this transfer is done indirectly bya soft blanket that is stamped by the plate. This soft blanket thenimpresses this image onto paper.

Color images are obtained by separating the image into four colorprocess plates, these four colors corresponding to Cyan, Magenta, Yellowand Black, commonly known as “CMYK”, that are then combined on paper. Ifan image is to be accurately represented, all four color separationsmust have the same length, scale and position when impressed on thepaper. However, this is difficult, as there may be misalignment andscale errors, these errors being classified as either fixed errors orpaper errors.

Fixed errors are errors in misalignment and scale that do not changefrom image to image. These errors are typically caused by tolerances ofthe prepress imaging devices, errors produced during development of theimaged plates, errors in mounting the plates on the printing presscylinders, and errors of the printing press mechanisms. Errors of theprinting press mechanisms typically include plate cylinders having losttheir roundness from wear or the like, and loosening of gears, bearings,etc., typically from wear over time.

Paper errors typically result from paper wetting by fountain solutionand ink, and by forces applied on the paper by the printing press, thattend to deform the paper. With the paper deformed, data is printed atundesired or unintended locations. Even when the deformation forces onthe paper are released, the paper does not usually recover to itsoriginal configuration, and thus, there is a difference between the data(on the imaged printing plate) and the resultant printed image.

Paper deformation effects, that result in paper errors, are explained byFIGS. 1A-1C Specifically, FIG. 1A shows the desired or ideal situationwhere a rectangular image 20, formed of lines 20a and 20 b, is to beimposed on the substrate 22, typically paper. In FIG. 1B, the paper 22has been impressed with a first separation, here a black (K of the CMYK)impression 24. This impression 24 is trapezoidal in shape (formed oflines 24 a, 24 b) as a result of the paper deformation in the press.Subsequently, in FIG. 1C, a second separation, for example a Cyan or “C”separation is impressed onto this deformed paper 22, as represented bybroken line 26, in a misregistration. Similarly in this manner, thesubsequent Yellow “Y” and Magenta “M” separations will also bemisregstered in accordance with the paper deformation.

In conventional press systems, some of the misregstration caused by thefixed errors was correctable mechanically by the operator afterreviewing the initially printed sheets. In this case, the operatormanually adjusted the relative positions of the printing plates orchanged pressure on the printed substrate.

These manual, operator-made adjustments have drawbacks. Initially, theseadjustments are time consuming and require considerable operator skill.Additionally, the adjustments of plate cylinders required expensivemechanical devices. Even in conventional presses, although the timerequired for adjustment is lessened, the adjustment machinery is morecomplex and more expensive.

A few presses have been designed such that printing plates are imaged onthe press, or “on-press”, whereby the plates are not removed from thepress for imaging. One exemplary press is the Model 74 KARAT offsetdigital press, manufactured by a Karat Digital Press of Herzlia, Israelin a joint venture with KBA (Koenig & Bauer Aktlengesellschaft). Thisdesign, besides reducing the make ready or preparation time, allowselimination of most of the registration fixed errors. However, papererrors remain and to date, a procedure does not exist for correctingmisregistration caused by paper errors.

SUMMARY OF THE INVENTION

The present invention improves the process of on-press printing member,typically printing plate, imaging, as it provides an automatic processfor plate preparation that compensates for registration and print-lengtherrors (plate loading is performed before imaging and therefore theposition accuracy is determined by the imaging system). The system ofthe present invention creates deformed images on the printing members,typically plates, during the imaging stage, these deformed images, beingsuch that the separations will be in register (coordinated) afterprinting.

The present invention automatically determines the exact position foreach data pixel within the distorted image to be placed onto the plates.This position is a sum of fixed errors and errors associated withstretch of the substrate, typically paper. The invention also providesan automatic procedure for predicting paper stretch errors.

The correction process uses the image as a measure of the distributionof the ink load and a set of fixed parameters (determined by the paper,ink and other press conditions) to solve a differential equation thatcalculates the errors associated with the paper stretch. These errorsare added to the fixed-errors map to obtain the final errors. The finalpixel-location map is deduced by inverting the process, namely findingthe position of a pixel such as when the position error is added to theposition set, the requested position on print will be obtained. Theresulting map is implemented, in conjunction with a “strobe” timing cardand/or data manipulation card, to create the distorted plate image,resulting in a printing plate being imaged such that misregistration ofthe separations forming the printed image on the substrate are minimizedor eliminated altogether.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be described with reference to theaccompanying drawings, wherein like reference numerals or charactersidentify corresponding or like components. In the drawings:

FIGS. 1A-1C are diagrams useful in explaining problems associated withthe prior art;

FIG. 2 is a schematic diagram of the imaging head and the strobe cardand associated systems in accordance with an embodiment of the presentinvention;

FIG. 3 is a flow chart detailing an embodiment of the present invention;

FIG. 4 is a diagram of a printed pattern where the ink load is uniformalong the Y-axis;

FIG. 5 is a plot of the registration error along the X-axis, inaccordance with the printed pattern of FIG. 4;

FIG. 6 is a diagram of a printed pattern where the ink load is uniformalong the X-axis; and

FIG. 7 is a plot of the registration error along the Y-axis, inaccordance with the printed pattern of FIG. 6.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is operable with printing presses, typicallydigital offset printing presses. For example, one such digital printingpress, that uses printing plates is commercially available as theQuickmaster D1, from Heidelberg Druckmaschinen AG of Germany. Thepresent invention utilizes digital offset printing deformation that isperformed during the stage of imaging the printing plates and/orcylinders.

The present invention utilizes a microprocessor or other computingdevice to automatically obtain relevant information as to the amount ofpaper distortion induced by the ink-load distribution in the image to beprinted, and then employs a strobe/data manipulation system, such asdetailed below.

Turning to FIG. 2, there is shown an exemplary imaging drum system 40.The system 40 includes an imaging drum 42, rotated via a shaft (notshown) by a motor 44, that is controlled by drum motor control 48. Anencoder 46 is coupled to the drum shaft, for supplying digital data tothe carriage control system 60, to the drum motor control 48 and to thestrobe card 72. An imaging carriage 50, controlled by the carriagecontrol system 60, is formed of a laser control card 52, and an opticalhead 54 which produces a high intensity laser beam 56 directed onto theimaging drum 42.

Carriage control system 60 is used to move the imaging carriage 50 alongthe drum 42. The carriage system 60 comprise a carnage motor control 62,carriage motor 64 and typically a ball screw 66, which translates therotational movement of motor 64 into translational movement along thedrum 42. Strobe card 72 locks on the frequency of the encoder signal 46,and generates a high frequency expose clock 74, that is in phase withthe rotation of the drum 42.

In normal operation, the ratio between the high frequency signal and theencoder signal 46 is kept constant. Also, the strobe card 72 is suchthat it can be preloaded with data from correction table 70. Data in thecorrection table 70 (typically in a data manipulation card) is such thatminor modifications of the ratio between the high frequency signal andthe encoder signal are made easily.

As a result of this modification the data can be expanded by slowingdown the expose clock 74, or compressed by raising the expose clock 74.By extending and compressing the data, corrections of the image platesis achieved in order to minimize and eliminate any misregistration ofthe color separations in a printed image. This is because the strobecard 72 and data correction table are such that they can change the rateof imaging, while the speed of the imaging drum 42 and the speed of thecarriage 50 are synchronized and kept constant.

FIG. 3 shows a flow diagram of the automatic correction scheme of thepresent invention. Initially, the image itself is typically in the formof a low resolution file, at block 102. Data as to the substrate and inkto be used, typically from a stored data base, is at block 104, andpress parameters, such as speed, impression pressure, ink temperaturesare at block 106. A fixed error map, at block 108, and parameters forthe process, block 110, form other elements for performing the presentinvention.

The low-resolution file, block 102, typically has a resolution of 1 dotper millimeter, and as such, is accurate enough for error evaluation.The low-resolution file may employ resolutions as great as 10 dots permillimeter.

One example of a low-resolution file is prepared by InkPro™, availablefrom Scitex Corporation of Herzelia, Israel. Turning now to the process110, it is designed to calculate the error of each pixel, at the lowresolution image, for each separation (typically separations 1-4,corresponding to CMYK, respectively), with respect to a first referenceseparation.

The process is directed to the relation between the errors and theimage, where, u^(i)(x,y) is defined as the misregister in thex-direction of the separation i at a point (x,y) on the paper. Here x isthe drum perimeter axis, y is the carriage screw axis. Only thex-component of the misregister is of interest here, since it is usuallymuch larger than that along the y direction. The magnitude of u^(i)(x,y)depends on (x,y) and i. This dependence is determined by the ink load onpaper f^(i)(x,y) of previous separations j≦i. The ink load plays therole of an error source in the equation describing the magnitude of themisregister. When calculating the ink load function, dot gain and thenon-linear response of the paper on ink load must be taken into accountby passing each image dot-area through a 1-D-LUT (look up table)correction.

The process is based on experiments showing the following behavior.Initially, a pattern was printed, that was uniform along the Y-axis(FIG. 4). Points 120 a, 121 a, 122 a and 123 a designate borders of theink area. Specifically, where the ink-load changes along the X-axis, theerrors of the different separations were found to be proportional to theintegral ink load, as plotted in FIG. 5, where points 120 b, 121 b, 122b and 123 b designate the ink borders corresponding to the points ofFIG. 4.

When a pattern was printed that was uniform along the X-axis (FIG. 6),namely, where the ink-load changes along the Y-axis, the errors of thedifferent separations were found to behave as plotted in FIG. 7.

This behavior is similar to that of a heat equation with sourceg_(i)(x,y), known in the art, which led to the assumption that theerrors u^(i)(x,y) follow a diffusion equation:${\frac{\partial}{\partial x}{u^{\prime}\left( {x,y} \right)}} = {{{- D}\frac{\partial^{2}}{\partial y^{2}}{u^{\prime}\left( {x,y} \right)}} + {g_{i}\left( {x,y} \right)}}$

Where D is the diffusion constant. It can be seen, that when theink-load g_(i)(x,y) does not change with y, namely g_(i)(x,y)=g_(i)(x),we obtain as a solution an integral (²/y² drops out), thus:u/x=g_(i)(x). Hence, u_(i)(x) = ∫_(U)^(Y)g_(i)(x^(′))  𝕕x^(′)

When y dependence is inserted, a “heat diffusion” process occurs thatcauses an error in the inked side of the paper to “spill” over into theclear areas.

When calculating the errors, each separation error is affected by thepreviously printed separations, according to the principle outlinedabove. Therefore:${g_{i}\left( {x,y} \right)} = {\sum\limits_{i \in j}\quad{A_{ij}{f^{\prime}\left( {x,y} \right)}}}$

When solving the equations, a constant matrix is used to describe theadditive effect, e.g., the error in separation 4 equals the sum oferrors in separations 1 to 3. Note that the ink load functionsf^(i)(x,y) are derived from the CMYK file values by transforming itthorough a 1-D LUT that takes into account the dot gain and a correctionfor the non-linear response of the misregister on the ink coverage onpaper.

The final equation using a vector notation is:${\frac{\partial}{\partial x}\left\lbrack {\overset{\_}{u}\left( {x,y} \right)} \right\rbrack} = {{{- D}{\frac{\partial^{2}}{\partial y^{2}}\left\lbrack {\overset{\_}{u}\left( {x,y} \right)} \right\rbrack}} + {\left\{ A_{ij} \right\}\left\lbrack {\overset{\_}{f}\left( {x,y} \right)} \right\rbrack} + {\left\lbrack {\overset{\_}{f}\left( {x,y} \right)} \right\rbrack{\left\{ B_{ij} \right\}\left\lbrack {\overset{\_}{f}\left( {x,y} \right)} \right\rbrack}}}$Here, u=(u^(K), u^(C), u^(M), u^(Y)) is the errors four-vector, andf=(f^(K), f^(C), f^(M), f^(Y)) is the image four-separation data (takinginto account the 1-D-LUT correction). Aij is he matrix, discussed above,describing the relation between the registration errors and the inkload, D is the diffusion parameter and Bij is a matrix describing secondorder corrections, taking into account interaction effects, related toink-over-ink areas. In most cases, these corrections are small, andtherefore Bij can typically be set Bij=0 for all i,j. The parametersAij, Bij and D are dependent on paper, ink, and machine parameters. Todetermine them, a calibration process is done, and the values of theparameters are saved in a database for future use.

The calibration process involves imaging and printing of “synthetic”files for a certain set of ink, paper and machine parameters. Theparameters Aij. Bij and D are adjusted until the measured errors areidentical (or within a small margin) to those predicted by the model.

A careful design of the “synthetic” files allows for faster and easiercalibration, by determining some of the parameters independently fromthe others. For example, the use of a full format, one separationuniform coverage simplifies the equation by removing the y-dependence,and the second-order interaction terms. Thus, it allows an easydetermination of some of the Aij's. Repeating the process with anotherseparation will give other Aij's until all of them are obtained. Then,the parameter D and the Bij can be determined by using more complexfiles.

To obtain the fixed error map, the following procedure is applied. Fixederrors are mapped on the plate surface, by imaging and printing the samegrid for all separations, and then measuring the relative shifts withrespect to the first separation on each point of the grid, to producethe fixed-errors map. The fixed-errors map is saved in memory, prior toimaging.

During operation, the user defines the printing variables beforestarting imaging the next job. The parameters, defined during thecalibration stage, are recalled from the database and loaded into thealgorithm or program of the process. The low-resolution file is loadedinto an array, and the differential equation is transformed into adifference scheme and solved numerically. The results give a goodestimate of the paper stretch errors, associated with the ink load.

When calculating the final errors, an interpolated fixed error map isadded to the image dependent errors to obtain the total error magnitudeas a function of position. The interpolation is required because thetypical resolution of the measured error map is about 0.1 dots permillimeter, while the image dependent errors are calculated on a 1 dotper millimeter grid. After the total number of errors is known, thestrobe/data system is optimized, as the strobe “timing” card 72 and/ordata manipulation card adjusts, expands or compresses (as detailedabove) the resultant image by controlling the rate of imaging (typicallyby controlling the laser beam 56 as detailed above), at block 112 (FIG.3). In this way, the residual errors after correction are minimal. Theimage, now “distorted” by above detailed process, is then placed ontoprinting members, typically plates, or other substrates, at block 114(FIG. 3).

The process is completely transparent to the user (and does not requireoperator intervention, except for the initial error mapping andpaper/ink calibration setup). It does not require expensive mechanicalapparatus, since the correction is distribution of done by asoftware/electronic hardware setting.

Although the preferred embodiment of the present invention has beendescribed in terms of digital offset printing, the method is applicableto all printing technologies, since it connects the geometrical errorsto the image printed. In particular, the technique is applicable toprepress imaging machines, provided the ink-paper calibration data isavailable.

The methods and apparatus disclosed herein have been described withoutreference to specific hardware of software. Rather, the methods andapparatus have been described in a manner sufficient to enable personsof ordinary skill in the art to readily adapt commercially availablehardware and software as may be needed to reduce any of the embodimentsof the present invention to practice without undue experimentation andusing conventional techniques.

It will further be appreciated by persons skilled in the art that themethods disclosed herein may be implemented by software or softwaremeans (data) executable on computing means, such as a CPU, PC, or othersimilar data processors, microprocessor, embedded processors,microcomputers, microcontrollers, etc. The computing means processes theinputted data from apparatus in communication therewith to calculate adesired result. Processing includes performing operations, preferably inthe form of programs or algorithms (as detailed above) for performingthe detailed methods of the present invention.

While preferred embodiments of the present invention have beendescribed, so as to enable one of skill in the art to practice thepresent invention, the preceding description is intended to be exemplaryonly. It should not be used to limit the scope of the invention, whichshould be determined by reference to the following claims.

1. A method for eliminating printing registration errors in a systemcomprising a processor for computing distortion parameters and animaging system in communication with said processor and configured forexposing distorted images, comprising the steps of: receiving input dataincluding paper data, at least one machine parameter and inkdistribution data; calculating image dependent errors from said inputdata; receiving at least one fixed error map dependent on machineparameters and obtained during a calibration run; predictingregistration errors based on the fixed error map and said imagedependent errors; and computing distortion parameters based on saidregistration errors for creating distorted images.
 2. The method ofclaim 1, additionally comprising: providing a printing member; andplacing said distorted image onto said printing member.
 3. The method ofclaim 1, wherein said step of computing includes providing a referenceimage, calculating errors for substantially all of the pixels for atleast one color separation in said reference image, and utilizing atleast one of strobe data or data manipulation card in combination withsaid calculated errors to control the rate of imaging to created adistorted image.
 4. The method of claim 3, wherein said step ofproviding a reference image includes providing said reference image in alow resolution file.